A blog about condensed matter and nanoscale physics. Why should high energy and astro folks have all the fun?

Friday, November 04, 2016

What is a metasurface?

As I alluded in my previous post, metamaterials are made out of building blocks, and thanks to the properties of those building blocks and their spatial arrangement, the aggregate system has, on longer distance scales, emergent properties (e.g., optical, thermal, acoustic, elastic) that can be very different from the traits of the individual building blocks. Classic examples are opal and butterfly wing, both of which are examples of structural coloration. The building blocks (silica spheres in opal; chitin structures in butterfly wing) have certain optical properties, but by properly shaping and arranging them, the metamaterial comprising them has brilliant iridescent color very different from that of bulk slabs of the underlying material.

Controlling the relative phases between
antennas in an array lets you steer radiation.
By Davidjessop - Own work, CC BY-SA 4.0,
https://commons.wikimedia.org/
w/index.php?curid=48304978

This works because of wave interference of light. Light propagates more slowly in a dielectric (\(c/n(\omega)\), where \(n(\omega)\) is the frequency-dependent index of refraction). Light propagating through some thickness of material will pick up a phase shift relative to light that propagates through empty space. Moreover, additional phase shifts are picked up at interfaces between dielectrics. If you can control the relative phases of light rays that arrive at a particular location, then you can set up constructive interference or destructive interference.

This is precisely the same math that gives you diffraction patterns. You can also do this actively with radio transmitter antennas. If you set up an antenna array and drive each antenna at the same frequency but with a controlled phase relative to its neighbors, you can tune where the waves constructively or destructively interfere. This is the principle behind phased arrays.

An optical metasurface is an interface that has structures on it that impose particular phase shifts on light that either is transmitted through or reflected off the interface. Like a metamaterial and for the same wave interference reasons, the optical properties of the interface on distance scales larger than those structures can be very different than those of the materials that constitute the structures. Bear in mind, the individual structures don't have to be boring - each by itself could have complicated frequency response, like acting as a dielectric or plasmonic resonator. We now have techniques that allow rich fabrication on surfaces with a variety of materials down to scales much smaller than the wavelength of visible light, and we have tremendous computational techniques that allow us to calculate the expected optical response from such structures. Put these together, and those capabilities enable some pretty amazing optical tricks. See here (pdf!) for a good slideshow covering this topic.

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About Me

My professional background: After an undergrad degree from Princeton in mechanical and aerospace engineering, I went to grad school at Stanford and got a doctorate in physics. Following a postdoctoral appointment at Bell Labs, I moved to Rice University and established a research program in experimental condensed matter physics, with a particular emphasis on nanoscale science. If you're interested in this stuff, please think about buying my book - it's a page-turner, and you'll want to finish it before the HBO miniseries spoils the ending. (That last part was a joke.) I blog regularly about science at Nanoscale Views. As should be obvious to pretty much everyone, anything I say there or here are my personal views, and in no way are official opinions of Rice University or its Department of Physics and Astronomy.